Process for depositing a metal layer upon a plastic

ABSTRACT

A metal layer is deposited onto a plastic body by admixing a soluble particulate filler and a metal which is reduceable to a catalytically active metal into a heat hardenable polymeric resin composition, applying heat to simultaneously harden the resin composition and to reduce the metal salt, removing at least one surface layer of the hardened polymeric composition to expose portions of the catalytically active metal particles and the soluble filler particles, which exposed filler particles are removed, and subjecting the treated surface to a metal plating bath for the electroless deposition of metal on the hardened polymeric surface by the catalytic activity of the metal particles.

United States Patent 1 [1.11 3,847,649

Sova 1 Nov. 12, 1974 [54] PROCESS FOR DEPOSITING A METAL 3,226,256 12/1965 Schneble, Jr. et a1. 1 17/212 LAYER P A PLASTIC 3,770,571 11/1973 Alsberg et a1 117/47 A [75] Inventor: Vladimir Sova,Mellingen,

Switzerland Assignee: Brown, Boveri & Cie, Baden,

Switzerland Filed: Mar. 9, 1973 Appl. No.: 339,935

[30] Foreign Application Priority Data Mar. 16, 1972 Switzerland 3929/72 References Cited UNlTED STATES PATENTS 8/1970 Powers et a1. 117/217 Primary Examiner-Ralph S. Kendall Assistant Examiner-Bruce H. Hess Attorney, Agent, or FirmOblon, Fisher, Spivak, McClelland & Maier [57] ABSTRACT A metal layer is deposited onto a plastic body by admixing a soluble particulate filler and a metal which is reduceable to a catalytically active metal into a heat hardenable polymeric resin composition, applying heat to simultaneously harden the resin composition and to reduce the metal salt, removing at least one surface layer of the hardened polymeric composition to expose portions of the catalytically active metal particles and the soluble filler particles, which exposed filler particles are removed, and subjecting the treated surface to a metal plating bath for the electroless deposition of metal on the hardened polymeric surface by the catalytic activity of the metal particles.

12 Claims, 1 Drawing Figure PROCESS FOR DEPOSITING A METAL LAYER UPON A PLASTIC I BACKGROUND OF THE INVENTION ing a metal layer to a plastic body, and more particularly to a method of preparing a surface of a polymeric resin body for electroless deposition whereby a thicker metal layer can be adherently bound to said body than has heretofor been possible.

2. Description of the Prior Art It is known to prepareplastic bodies for electroless deposition by providing an embedded dissolvable filler at the surface of the body, which is dissolved out prior to electroless deposition. The purpose of the surface treatment has been to provide anchoring sites for the applied metal layer.

It has been found, however, that the thickness of the metal layer which can be adherently formed in this manner is substantially limited. If the metal layerbecomes too thick, there is a tendency for local chipping and peeling of the layer from the substrate, and there is atendency for mechanical strains and bubbles to develop in the deposited metal. In general, the maximum thickness which was obtainable by that prior art method, with any degree of reliability, has been about 30 microns.

Accordingly, a need exists for a technique whereby a thick deposit of electrolessly formed metal can be formed adherently onto the surface of plastic bodies.

SUMMARY OF THE INVENTION Accordingly, it is one object of this invention to provide a novel technique of adherently applying a layer of electroless metal to a plastic substrate.

It is further an object of this invention to provide a technique for electroless deposition onto a polymeric resin body.

These and other objects of this invention, as will hereinafter become more readily apparent have been attained by admixing into an heat hardenable polymeric resin composition, a dissolvable filler, which is insoluble in said composition, and a metal salt which is catalytic to electroless deposition in its reduced state, heat hardening said resin composition and simultaneously reducing said metal salt to its catalytically active state, removing a surface layer of the cured body so as to expose a portion of said reduced salt and a portion of said filler, dissolving out the exposed filler, and subjecting said treated surface to an electroless bath to effect an electroless deposition of metal due to the catalytic activity of said reduced salt.

DETAILED DESCRIPTION OF THE INVENTION The particular polymeric resin composition which is treated in the present invention is not critical and any thermosetting or thermoplastic resin may be used. The only limitation is that the resin must be sufficiently fluid in the unhardened condition to permit admixing of the metallic salt and particulate filler therewith. The composition may contain a curing or hardening agent, or may be formed from a monomeric composition which is heat polymerizable into a hardened composition. Any of the widely used epoxy resin compositions, olefin resin compositions, carbonate resin compositions, acrylic resin compositions, formaldehyde resins, polyester resins, and the like may be used so long as they are heat hardenable.

The metal salt used herein is of the type which is catalytically inactive until subjected to reducing reaction. The reduced state may be a reduced metal salt, but usually will be a metal in its 0 valence state. Suitable metal salts in general are those of the Group VIII and Group IB elements. The metals are preferably bound to organic anions. For instance, suitable metal salts include silver lactate, iron citrate, gold acetate, and the like. The advantage of metal-organic anion salts is that they are easily reduceable within the usual hardening temperature range of most plastics, and the anion portion will not have an adverse affect on the properties of the plastic. Of course, there are a large number of metallic salts which can be heat reduceable to a catalytic state for electroless deposition, and the list is too numerous to mention, but would be readily apparent to one of ordinary skill in the art.

The metal salts need be used only in an amount sufficient such that when a surface layer of the plastic :body is removed, a sufficient quantity of the reduced metal salt will be present to catalyze an electroless metal plating from an electroless bath.

The metal salt may be admixed uniformly with the unhardened plastic, or it may be sprinkled on the surface of the unhardened plastic, contained in a suitable mold.

The metal salts can be added to the unhardened plastic in the form of a finely pulverized solid. Goodresults are obtainable by grinding both the metal salt and the filler in solid state or by grinding the solids in a disper-. sion in a volatile medium with subsequent evaporation of the latter. Another means of forming a fine particle size is by soaking a carrier with a metal salt solution and by evaportion of the solvent and pulverization of the dried mixture, a small grain particle size can be obtained.

The quantity of metal salt should be about 1 to 50 parts by weight of the metal salt acting as a catalyst per 99 to 50 parts by weight of the carrier. The carrier may be selected such that it is soluble in the electroless deposition metal bath. Suitable carriers for the metal salt include the amino polycarbonic acids, oxycarbonic acids and the like.

A wide variety of dissolvable particulate fillers may be used, and particularly preferred are those fillers which are known to enhance the properties of the plastic body. There is no criticality in the particular filler used except it must be easily dissolvable from the plastic composition without substantial damage to the plastic body. The filler may be solvent soluble or acid or base soluble depending upon the characteristics of the particular resin composition.

Suitable fillers have been found to be tartaric acid, citric acid, hydroxyethylene-diamine-tetra-acetic acid, and potassium tartarate. These fillers have been found to be quite suitable since they are easily dissolvable in water, sodium hydroxide and potassium hydroxide.

Also suitable are such fillers as ethylene glycol-bisaminoethyl ether-tetra acetic acid, 1,2-diamino-cyclohexane-tetra acetic acid, ethylene diamine-tetra acetic acid, diethylene-triamine-tetra acetic acid, hydroxyethylene-diamine-triacetic acid and ethylene-diaminedi(0-hydroxy-phenyl)-acetic acid.

In the present invention a finely distributed catalytic salt in its inactive state is added to the plastic in the unhardened state. The plastic is then hardened and simultaneously the salt is activated during hardening of the plastic by reduction of the metal salt to its catalytically active state, usually the free metal. By this technique, thick metal layers may be deposited by subsequent direct metal precipitation onto the plastic without adversely affecting the physical or chemical properties of the plastic or the metal layer. Moreover, metal deposits of even greater than 30 microns can be adherently deposited onto the plastic surface. The adhesion achieved according to this process appreciably exceeds that prescribed by the DIN 40,802 standards, i.e., pull-off of 3.2 kg. one-inch/width of sample. Adhesive values of 25 kg for corresponding sample widths have been achieved.

Another advantage of the present invention is that the surface of the plastic is enriched with catalyst. That is, a higher concentration of catalyst forms at the boundary areas of the plastic, than within the plastic, so that an active surface layer is obtained. This layer subsequently initiates a further chemical metal reduction during the electroless metal deposition process.

According to one preferred example embodiment of this invention, soluble filler may be dissolved by the medium of a copper bath. This entails the advantage that copperizing of the surface will only occur when the filler exposed at the surface is fully dissolved out so that the adhesion and the ductility of the separated metal layer will be appreciably increased.

in order to further clarify the invention reference is made to the FIGURE wherein like reference numerals in the drawing relate to like reference numerals in the following discussion.

The drawing shows a cross-section through a plastic body, the surface of which is coated with a metal layer 1. In this instance, the metal is a nickel layer which has been galvanically deposited onto a copper layer 2. The copper layer 2 has been deposited onto the synthetic resin 3 so as to espouse its shape. The synthetic resin 3 has been reinforced by carbon fibers 4. The void spaces 5 had been formed by dissolving the filler from the exposed surface of the plastic. Unexposed filler 6 is. left undissolved in the plastic body. Metal particles 7 reduced from the metal salt are embedded between the grains of the filler and copper layer 2, with increasing concentration towards the latter. These metal particles 7 act as the catalyst for precipitation from an electroless deposition bath.

Having now fully generally described the invention, a further understanding can be attained by reference to certain specific examples which are included herein for purposes of illustration only and are not intended to be construed as limiting unless otherwise so specified.

EXAMPLE 1 A plastic consisting of epoxy resin 100 gm, hardener gm, and acetone 20 gm, was mixed in the liquid state with 50 gm of powdered tartaric acid acting as the filler. Then 5 gm of silver lactate was added in finely distributed form and the synthetic substance so mixed was evenly deposited on a nonmetallic molding or shaping part X 200 X 5 mm so that a layer of about 20 microns thickness was obtained. The mold was heated to 120 C. for half an hour so as to harden the plastic layer. The silver lactate was reduced to powdered silver.

Subsequently the surface was treated by chemical etching with chromic acid in order to dissolve out the exposed filler powder grains at the surface of the plastic boxes. The form or mold was rinsed in water and introduced into a copper electroless deposition bath for the purpose of effecting electroless coppering. The bath composition was as follows:

copper sulfate 7.5 gm/liter ethylene diamine tetra-acetic 15.0 gm/liter acid potash lye 20.0 gm/liter sodium cyanide 0.4 gm/liter formaldehyde 20 ml/liter Most of the alkali-soluble salt was dissolved in this copper bath from the synthetic resin, the remaining silver again activating the copper separation, thus taking part in the coppering as a catalyst.

A nickel layer, acting as an additional protective coat was deposited onto the coppered mold in a galvanic metal bath of the following composition:

nickel sulfate 300.0 gm/liter nickel chloride 60.0 gm/liter boric acid 40.0 gm/liter Arbitrary thicknesses were found to be feasible after a copper layer of 30 microns thickness had been deposited within the shortest possible time in the copper bath. There was found to be no changes in the structure or in the properties of the fundamental plastic body.

EXAMPLE 2 A plastic consisting of:

Eolyester resin 100 gm ardener 20 gm acetone 20 gm in the liquid state was mixed with 10 gm of powdered ethylene-diamine-tetra-acetic acid and with 1 gm of gold acetate. This mixed synthetic substance was deposited onto a non-metallic mold or form of glass-fiber reinforced plastic, so that a coat of about 20 microns thickness was achieved. The mold was then heated to 100 C. for half an hour so as to harden the synthetic layer. Then the surface was treated with emery paper to free the powder grains of the filler and of the metal which were located in the region of the surface layer. Electroless coppering as described in Example 1 was performed by inserting the mold or the plastic body into a copper bath of the following composition:

copper sulfate 7.5 gm/liter ethylene-diamine-tetra- 15.0 gm/liter acetic acid potash lye 20.0 gm/liter sodium cyanide 0.4 gmlliter formaldehyde 20 ml/liter The catalytic action of the metal, initiated by the additional reduction in the copper bath, enabled a precipitation of a copper layer which was thicker than 30 mi crons of the synthetic body within the shortest time.

EXAMPLE 3 Asynthetic substance was mixed in the liquid state from the following ingredients:

epoxy resin 100 gm hardener 20 gm acetone 20 gm This synthetic substance then was deposited onto a nono-metallic mold of a fiberglass reinforced plastic, and in an even manner, so that a layer of about 20 microns was formed. The mold was heated to 90 C. for about half an hour to obtain partial hardening of the epoxy resin, until a gelled state of the deposited synthetic substance had been achieved. The surface of this synthetic substance upon which the metal layer was deposited was dusted with a powdered mixture of 20 gm of powdered tartaric acid and 2 gm of iron citrate. The mold so treated was then fully hardened for 4 hours at 180 C. Subsequently the hardened surface was treated with emery paper in order to release the powder grains located in the region of the surface. After rinsing in water, the mold was dipped in an electroless nickel deposition alkaline bath of the following composition:

nickel chloride 30 gm/liter sodium hypo-phosphite 10.0 gm/liter sodium citrate 100.0 gm/liter ammonium chloride 50 gm/liter Ammonium was added until the pH value was between I 8 10. A nickel layer 15 mm thick was deposited from a galvanic nickel bath of the following composition:

nickel sulfate 300 gm/liter nickel chloride 60 mg/liter boric acid 40 gm/liter Having now fully described the invention, it will be apparent to one of ordinary skill in the art, that many changes and modifications can be made to the invention, without departing from its spirit or scope.

Accordingly, what is new and intended to be covered by Letters Patent is:

l. A process for the deposition of a metal layer onto a plastic body which comprises:

a. admixing a solvent, acid, or base soluble particulate filler, and a catalytically inactive reduceable Group VIII or IE metal salt of the Periodic Table into a curable polymeric resin composition selected from the group consisting of epoxy resins,

olefin resins, carbonate resins, acrylic resins, formaldehyde resins, and polyester resins;

b. applying heat-so as to effect curing of said polymeric resin composition to form a plastic body, and to simultaneously reduce said metal salt to an active catalytic state for subsequent electroless deposition;

c. removing at least one surface layer of said cured composition so as to expose a portion of said soluble filler and said reduced metal salt; and

d. desolving out said exposed filler under conditions which do not dissolve said cured polymeric resin and subjecting said treated surface to an electroless deposition from an electroless deposition bath by the catalytic activity of said reduced metal salt.

2. The process according to claim 1, wherein the concentration of the metal salt is greater at the surface of the plastic body than within said body.

3. The process according to claim 1, wherein the powder grains of the metallic salt is reduced to the metallic state so as to initiate a further chemical metal reduction during electroless deposition.

4. The process according to claim 1, wherein the dissolvable filler is dissolved out by contact with a copper bath capable of dissolving said filler.

5. The process according to claim 1, wherein the metallized plastic is further inserted into a galvanic bath in order to achieve a second metallic coating 6. The process according to claim 1, wherein the Group VIII or IB metal salt is silver lactate, iron citrate or gold acetate.

7. The process according to claim 1, wherein the polymeric resin is an epoxy resin.

8. The process according to claim 1, wherein the polymeric resin is a polyester resin.

9. The process according to claim 1, wherein the filler is an amino-poly-carboxylic acid or a salt thereof.

10. The process according to claim 1, wherein the tiller is an oxycarboxylic acid or salt thereof.

11. The process according to claim 1, wherein the polymeric resin composition is completely hardened prior to the removal of said surface layer.

12. The process according to claim 1, wherein copper or nickel is electrolessly plated onto said plastic body. 

1. A PROCESS FOR THE DEPOSITION OF A METAL LAYER ONTO A PLASTIC BODY WHICH COMPRISES: A. ADMIXING A SOLVENT, ACID, OR BASE SOLUBLE PARTICULATE FILLER, AND A CATALYTICALLY INACTIVE REDUCEABLE GROUP V111 OR 1B METAL SALT OF THE PERIODIC TABLE INTO A CURABLE POLYMERIC RESIN COMPOSITIONS SELECTED FROM THE GROUP CONSISTING OF EPOXY RESINS, OLEFINS, CARBONATE RESINS, ACRYLIC RESINS, FORMALDEHYDE RESINS, AND POLYESTER RESINS: B. APPLYING HEAT SO AS TO EFFECT CURING OF SAID POLYMERIC RESIN COMPOSITION TO FORM A PLASTIC BODY, AND TO SIMULTANEOUSLY REDUCE SAID METAL SALT FO AN ACTIVE CATALYTIC STATE FOR SUBSEQUENT ELECTROLESS DEPOSITION; C. REMOVING AT LEAST ONE SURFACE LAYER OF SAID CURED COMPOSITION SO AS TO EXPOSE A PORTION OF SAID SOLUBLE FILLER AND SAID REDUCED METAL SALT; AND D. DESOLVING OUT SAID EXPOSED FILLER UNDER CONDITIONS WHICH DO NOT DISSOLVE SAID CURED POLYMERIC RESIN AND SUBJECTING SAID TREATED SURFACE TO AN ELECTROLESS DEPOSITION FROM AN ELECTROLESS DEPOSITION BATH BY THE CATALYSTIC ACTIVITY OF SAID REDUCES METAL SALT.
 2. The process according to claim 1, wherein the concentration of the metal salt is greater at the surface of the plastic body than within said body.
 3. The process according to claim 1, wherein the powder grains of the metallic salt is reduced to the metallic state so as to initiate a further chemical metal reduction during electroless deposition.
 4. The process according to claim 1, wherein the dissolvable filler is dissolved out by contact with a copper bath capable of dissolving said filler.
 5. The process according to claim 1, wherein the metallized plastic is further inserted into a galvanic bath in order to achieve a second metallic coating
 6. The process according to claim 1, wherein the Group VIII or IB metal salt is silver lactate, iron citrate or gold acetate.
 7. The process according to claim 1, wherein the polymeric resin is an epoxy resin.
 8. The process according to claim 1, wherein the polymeric resin is a polyester resin.
 9. The process according to claim 1, wherein the filler is an amino-poly-carboxylic acid or a salt thereof.
 10. The process according to claim 1, wherein the filler is an oxycarboxylic acid or salt thereof.
 11. The process according to claim 1, wherein the polymeric resin composition is completely hardened prior to the removal of said surface layer.
 12. The process according to claim 1, wherein copper or nickel is electrolessly plated onto said plastic body. 